HONG KONG relatives of passengers killed in last week's Aeroflot disaster have demanded to know whether a boy was at the captain's controls and a steward in the co-pilot's seat before the crash in Siberian wasteland. Ten Hong Kong men and women, in Moscow after holding rites for loved ones at Novokuznetsk near the crash site, joined yesterday in questioning senior Aeroflot staff about who was flying the Airbus A310 when it nose-dived. Aeroflot's Moscow general director as well as the beleaguered airline's commercial director were asked: ''How could a boy be allowed to fly the plane?'' Investigators suspect the 15-year-old son of second pilot Yaroslav Krudinsky was at the captain's controls and flicked the wrong switches while a male steward was in the co-pilot's seat. Barbiturates, sometimes used to treat epilepsy, were reportedly found in the boy's blood. But the relatives' questions have gone unanswered by their hosts Aeroflot, who have refused to confirm black box evidence pointing to one of the most extraordinary disasters in civil aviation history, in which all 75 on board were killed. ''We told the family members today that reports about a boy in charge of the plane are not official,'' Paddy Chung Chi-kan, a Hong Kong-based Aeroflot official who escorted relatives to Russia, said yesterday. The 10 Hong Kong people returned to Moscow from Novokuznetsk early yesterday after performing the grisly task of trying to identify the bodies, many of them charred and disfigured almost beyond recognition. Two bodies were positively identified and plans were being made yesterday for one to be cremated in Moscow and the other to be transported back to Hong Kong for burial, but the other four Hong Kongers killed could not be identified.